Unlike a cathode-ray tube, generally, a liquid crystal display (LCD) device does not have a light emitting function in itself, and thus it is required that a light emitting device is maintained at a uniform brightness over an entire screen.
According to the methods of providing the light source, LCDs can be classified into a transmissive type, in which a separate light and a backlight unit are used, and a reflective type in which an external light is used as the light source. Of these methods, in the case of reflective type LCDs, many studies have been carried out because the backlight unit is not required and their power consumption is low. However, many applications have not been made until now since their visibility is low when the brightness is not sufficient from the external light source. On the other hand, in the case of transmissive type LCDs, which have been actively used in recent years, the key factor is to supply the light source with a uniform brightness through a backlight unit.
A backlight unit can be classified into a top-down method system, in which a light source is placed at the bottom surface of a liquid crystal panel to illuminate the entire surface of the substrate, and an edge illumination system, in which a light source is placed at both side surfaces of the unit and light is evenly diffused through a light guide plate and a reflection plate.
Such an edge illumination system backlight unit is mainly used for a small-sized LCD monitor or notebook computer since the brightness is uniform and the power consumption is low, but a light guide plate is definitely required for evenly diffusing the light from the side surfaces.
In the case of the top-down method system backlight unit on the other hand, the light use rate is high because a light source directly illuminates a substrate, and it is applicable to a large-sized LCD TV or monitor because the size is not limited. However, it causes a problem of increased heat, as a light source is very closely positioned with a liquid crystal panel and a large number of lamps are required to supply the light source. In the case where the heat produced is too high, it can be the main reason for inducing a smudge on the screen, thereby shortening the life of a liquid crystal panel. In recent years, particularly as LCDs have become larger and thinner, the thermal problems of the backlight unit have emerged as a problem to be solved by all means.
The structure of a typical top-down method system backlight unit is illustrated below.
In the top-down method system back light unit, a supporting rod for holding a liquid crystal panel is positioned around a lamp, that is, a light source, and also a diffuse sheet, a prism sheet, and a dual brightness enhancement film (DBEF) sheet are sequentially arranged at an upper portion of the lamp. Furthermore, a reflection plate for preventing the light from leaking, an external supporting rod, and a lower plate functioning as a heat sink are arranged at a lower portion of the lamp.
As a reflection plate material for the components of such a backlight unit, a white polyester film is disclosed in Japanese Laid-Open No. H04-239540; however, there is a problem in that the tint is lowered and the luminance is reduced due to yellowing of a reflection plate caused by the heat generated from a light source.
A technology for adding various additives and a structural change to improve the reflectivity, transmittance ratio, or the like, of a white polyester film is disclosed in Japanese Laid-Open Nos. 2002-98811, 2002-138150, and 2001-305321, and a technology for using a white porosity polyester film to enhance the reflectivity of a reflective film is disclosed in Japanese Laid-Open Nos. 2002-50222 and 2002-40214. Moreover, a technology for manufacturing a reflection plate using an ultra-fine foam polyester sheet is disclosed in Japanese Laid-Open Nos. 2003-145657 and 2003-121616.
A technology for the reflectivity and shock resistance of a white polycarbonate resin is disclosed in U.S. Pat. No. 5,837,757, Japanese Laid-Open Nos. H07-242781 and H09-176471, and a technology for manufacturing a reflective sheet using a flame-retardant white polycarbonate resin is disclosed in Japanese Laid-Open No. 1999-181267.
However, in the technology for manufacturing a reflection plate as described above, a method is disclosed for improving the reflectivity, heat resistance, shock resistance, or the like, of a reflection plate material, but a solution to the thermal problem of a backlight unit is not presented. In particular, any attempt at solving the thermal problem by applying a thermal conductive resin composition to a reflection plate material has not been made.
A method for applying a high thermal conductive metallic material such as aluminum to a lower plate of the backlight unit is disclosed to solve the thermal problem of the backlight unit in Korean Laid-Open No. 2004-0017718, but there is a problem in that a solution to the thermal problem is obstructed since a reflection plate is positioned between a heating element and a lower plate, and the cost of a product increases.
In order to solve the problems occurring in the prior art as described above, it is an object of the present invention to provide a reflection plate for a backlight unit in a liquid crystal display device for effectively solving the thermal problem of the backlight unit, and having excellent properties such as shock resistance, heat resistance, mechanical strength and the like, as well as having excellent reflectivity, thereby improving the durability of the liquid crystal display device.
Moreover, it is another object of the present invention to provide a backlight unit of a liquid crystal display device for effectively solving the thermal problem of the backlight unit, and simplifying the manufacturing process.